Residential Heat Best Practices IFurnaces and Boilers

Skip HaydenAdvanced Combustion Technologies

Better Buildings by Design 2008Burlington, VTFebruary 2008

Natural ResourcesCanada

Ressources naturellesCanada

Advanced Combustion Technologies

Objectives• Discuss space & water heating technologies

for oil and gas• Understand what makes heating systems

efficient• Appreciate advantages & disadvantages of

various new “high efficiency” systems• Be better able to assess suitability of newer

technologies for comfort and efficient operation in new or existing housing

What are youlooking for

??

??

? ?

What is your prime energy choice ?

• oil • natural gas• propane • wood • electricity

Heat Distribution ?

“Conventional”HydronicHeating

Radiant Floor ?

Warm Air System

Furnace

Fan Coil (Hydro-Air)

LOCAL LOCAL SPACE SPACE

HEATING ?HEATING ?

What makes a combustion system energy-efficient and

where is energy lost ?

Burner Furnace/Boiler/HeaterFurnace/Boiler/Heater

Chimney

Combustion System

Fuel

Flue Gas

Air

Dilution AirCool Feed

In

Hot Supply

Out

Casing Loss

Burner Furnace/Boiler/HeaterFurnace/Boiler/Heater

Chimney

Combustion System

Fuel

Flue Gas

Air

Dilution

AirCool Feed

In

Hot Supply

Out

Casing Loss

To appreciate what makes today’s heating technologies work (and not), it will be useful to appreciate some Combustion System Principles

Fuels

• No. 2 oil• Natural gas• Wood• Propane• + electricity –

fans/blowers, pumps, …

FUEL PROPERTIES(Ultimate Analysis)

• Carbon • Hydrogen• Sulphur• Nitrogen• Oxygen• Ash (Na, K, Fe, heavy metals Hg, )• Heating Value

Ultimate Analysis & HHV for Different Fuels(Dry Basis)

Property No. 2 Oil Nat Gas Propane Wood Pellets

Carbon 86.4% 71.6 81.7 49.7 49.8

Hydrogen 13.3% 23.2 18.3 5.8 5.9

Sulphur <0.3% trace 0.02 0.02

Nitrogen 4.3 0.08 0.08

Ash 0.3 0.1

Oxygen 0.9 44.1 44.1

Moisture 17-60 3HHV, Btu/lb 8000 8100

HHV , Btu/… 140k/gal 1007/ft3 92.7k/gal 17-27M

/fullcord 336/bag

Combustion Reactions

C + O2 CO2

4H + O2 2H2OS + O2 SO2

Oxygen is required for combustion

“Air”

• Nitrogen @ 79.1%• Oxygen @ 20.9%

Stoichiometric Airthe chemically exact amount of air

(oxygen) to achieve complete combustion of C, H & S

In order to ensure satisfactory combustion with minimal incomplete combustion, we need more air, what we call …

Excess Air

Amount of air greater than stoichiometricrequired for satisfactory combustion

Characterized by CO2 or O2 in flue gas

Excess Air = Total Air – 100%

Efficiency ? ? ?

First, a misused term …

Combustion Efficiency

Measures how completely the fuel is burned to CO2 & water vapour

Incomplete Combustion Products (Pollutants)

• Carbon (particulates, soot)• Hydrocarbons (CH4, VOCs, PAH, …)• Carbon Monoxide• Others

Combustion Efficiency• For nearly all oil and gas combustion systems,

combustion efficiency is >99.5%• For these, there is effectively no energy lost

directly due to incomplete combustion• Incomplete combustion can result in soot

deposits, which can insulate the heat exchanger over time and drop efficiency

• Older airtight woodstoves and many central woodburning central boilers/furnaces and fireplaces can have combustion efficiencies ~90%, resulting in major pollutant emissions and low overall efficiencies

Most times, when we think about efficiency, we are concerned with how effectively we are using the available energy in the fuel

Steady State Efficiency

• Usually the most efficient point of operation

• Like cruising on the highway

• Many smaller systems and certain industrial processes rarely operate in steady state

• Gives a base point of reference and guides system readjustment to compensate for performance degradation or to modify to increase performance

Equilibrium:Steady State Operation

Transient Efficiency

Many combustion systems (nearly all residential/commercial ones) do not operate in steady state for most of their operation.

They cycle up and down, taking a significant time to reach equilibrium, if at all.

In general, transient systems are significantly less efficient than ones that operate in steady state.

Equilibrium:Steady State Operation

Transient Operation

Transient/Cyclic Operation

Excess Air

The extra amount of air above that theoretically needed for

combustion, in order to actually get satisfactory burning

Effect of Excess Air

on Flue Gas Temperature

and Efficiency

Decreasing Excess Air

Efficiency

Effect of Excess Air on

Flue Gas Temperature

and Efficiency

Decreasing Excess Air

Decreasing excess air:• Lowers air demand• Reduces mass flow and velocity

thru the system• Improves heat exchange• Lowers flue gas temperature• Increases efficiency !

Cycling & Post-Purging

• Operates on a fixed time length (say 30 sec) each time after the burner shuts off, to ensure combustion gases are evacuated from the system

• Removes fixed am’t of heat from unit each cycle• Loss is about 1.5% for a typical number of cycles

per year (~2000 for normal furnace)• If purge length is doubled, loss is doubled;

similarly if no. of cycles doubles, loss doubles• If unit short-cycles and number of cycles

increases to 10 000 (not uncommon) loss goes up 5x to 7.5%; to 20,000 means loss up to 15%

AT01-2, 6hr WH

020406080

100120140160

9:50

10:0

510

:20

10:3

510

:50

11:0

511

:21

11:3

611

:51

12:0

612

:21

12:3

712

:52

13:0

713

:22

13:3

713

:52

14:0

714

:23

14:3

814

:53

15:0

815

:23

15:3

9

Time

Tem

p [C

]

01020304050607080

Con

c [%

]

Flue GasO2CO2

Casing LossStack Loss

Factors Affecting Overall Energy Consumption

Demand SupplyHeat Req’d Heat Losses

Outdoor Temp Transmission S.S. EfficiencyThermostat Infiltration Transient

Off-Cycle Comb & Dil Air(near,long)

Electricity (FD/ID fans)

Circ fan/pump)

Seasonal or AnnualFuel Utilization Efficiency

(AFUE)

This is a cumulative (average) efficiency of a system over a year or other extended period in response to varying conditions

It works quite well for warm air furnaces, but can misrepresent efficiency of some boiler technologies and of integrated space/water heating systems

Carbon & Hydrogen in Fuels

0

10

20

30

40

50

60

70

80

90

Wood No.2 Oil Propane Natural Gas

HydrogenCarbon

% by Wt

Natural Gas has high hydrogen & a low level of sulphurOil has significant sulphur + part.

Hydrogen Losses for Different Fuels

0

2

4

6

8

10

12

LigniteWoodNo.6 OilNo.2 OilPropaneNat Gas

Energy ContentsHeating Oil 140 kBtu/galNatural Gas 1 007 Btu/ft3

100700 Btu/thermPropane 92.7 kBtu/galElectricity 3413 Btu/kwhMixed Hardwood 26 Mbtu/fullcordWood Pellets 336 kBtu/bag or

16.8 MBtu / tonCoal 24 Mbtu / ton

Oil-FiredSystems

Combustion Air

Dilution Air

Flue Gas

Most crucial elementto achieve

good performanceand high efficiency

is theoil burner

Retention Head Burner

Retention Head Oil Burner• Better fuel-air mixing (EA - 50%)• Fair resistance to stack and house

pressure fluctuations/depressurization• Resists off-cycle flow loss• Firing rate down to 70kBtu with good EA• Efficiency should be 80-83%• Should have delayed action solenoid valve

to reduce soot/degradation

High Static Burners

Hi Static Burner

Decreasing Excess Air

High-Static Oil Burner• Excellent fuel-air mixing (EA - 25%)• Stable under stack (exhaust) pressure

fluctuations• High resistance to house depressurization• Zero off-cycle flow loss (ex post-purge)• Firing rate down to 70kBtu with good EA• Efficiency should be 82-86%• Minimal sooting/degradation• Should be burner of choice

Two-Stage Burner• Excess air must be controlled for both rates• If flue gas minimum temperature is governed by

low fire, then high fire condition will be inefficient• If the reverse, or a compromise, danger of

corrosion of tail end of unit or in venting system• If furnace, we see no evidence of energy savings

with 2 stage operation• Not convinced of benefits, esp just at 70% &

100%

Air Demands

• Cast iron head burner 100% excess air• Retention head burner 50% excess air• High static burner 25% excess air• Barometric damper 1-5 times air for

combustion• Combustion air related to air change for

1500 ft2 two storey + 750 ft2 basement = 18000 ft3 house

Combustion Air for Oil Burners

(70 kBtu/h; 18 k ft3 house)

0

0.02

0.04

0.06

0.08

0.1

0.12

ConventionalCast Iron

Retention Head

Hi-Static Oil,NO Dilution

Combustion & Dilution Air for Oil Burners

(70 kBtu/h; 18 k ft3 house)

0

0.05

0.1

0.15

0.2

0.25

0.3

ConventionalCast IronRetention Head

Hi-Static Oil, NODilutionHigh Dilution

Low Dilution

No Dilution Air

“Direct Vent” Oil Furnace

• High static burner• No dilution device• Low air requirements• No off-cycle loss• Minimal flue travel (insulated pipe)• Can be sealed combustion• Potential for high efficiency (~86%)

What about an Oil-Fired Condensing Furnace ?

Dewpoints for Different Fuels

60708090

100110120130140

0 50 100

Natural GasPropaneNo. 2 Oil

oF

Excess Air, %

Condensing Oil Furnace

• Much less energy tied up in latent heat than gas

• Dewpoint is low, but return air such that condensing occurs

• Condensate is very acidic, due to S in oil – best to use low S oil

• Heat exchanger must be VERY hi grade – Al-29-4C

• Sooting can result in “acid smut” which increases corrosive action – use good high-static burner

Oil Boilers

Condensing Oil-fired Boiler

High efficiency possible by recovering latent heat from water vapour in flue gas.

Similar issue to condensing furnaces re potential for latent heat recovery, and corrosion potential; low S oil better much better

Typical return water temperature (except for radiant floors) is above dewpoint, so make every effort to lower it: outdoor reset, lower flow rates, preheat cold service water

Condensing for Part of the Season

2030405060708090

100

1 1.5 2 2.5

Baseboard oversize factor

Part

of h

eatin

g se

ason

boi

ler

can

oper

ate

in c

onde

nsin

g m

ode

(%)

Gas - Minneapolis

Gas - Long Island

Oil - Minneapolis

Oil - Long Island

Service (Tap) Water Heating

Combined Space-Water Heating

However, this does not mean:“Combo” Conventional Tank Water

Heater with Fan Coil• Relatively cheap• High flue gas

temperature• Reduced life• Inefficient • Much lower AFUE

than furnaces, because standard does not apply

• Not recommended !

Tankless Coil Boiler or Tank-within-Tank Boilers

• Very inefficient summer and overall operation

• Large number of cycles with inherent inefficiency and sooting

• Not recommended !!

However, well-conceived combined systems for space and water heating can be an effective way around the technical limitation of oversizing with oil-fired equipment in energy efficient housing.

Efficient Oil-Fired Space/Water Heating

Systems

Gas-FiredSystems

Conventional Naturally-Aspirating Gas Furnace

Dilution Air

Combustion Air

Conventional Gas Furnace• Naturally aspirating • Highly susceptible to depress &

spillage• Draft hood (extra heated air loss)• Continuous pilot (energy waste)• Large off-cycle loss• Low seasonal efficiency ( ~ 60% )• Now an “antique” !

Mid-Efficiency

Gas Furnace

Mid-Efficiency Gas Furnace• Powered exhaust• Electronic ignition• No dilution air• So-so resistance to depressurization• Probs with side-wall vent -No plastic!• Should NOT be efficient ( <81% )• Oversizing loses efficiency and gives short-

cycle condensation/corrosion problems with furnace or vent

High EfficiencyCondensing

Furnace

*****

Dewpoints for Gaseous Fuels

60708090

100110120130140

0 50 100

Natural GasPropane

oF

Excess Air, %

High-Efficiency Gas Furnace• Powered exhaust• Electronic ignition• No dilution air• High resistance to depressurization• Excellent side-wall vent - plastic!• Should be well above 90% • Efficiency improves with slight oversizing

Effects of Brushless DC Motors in Fan/Motor Sets on Electrical Use/Gas Consumption in Heat/Ventilation/Air

Conditioning Distribution

Gas Consumption with ECM & PSC Motors

Electricity Consumption& Air Flows

Low Speed Circulation

High Speed Heating

ECM 16.5 w @448 cfm

284 w @1215 cfm

PSC 350 w @960 cfm

490 w @1317 cfm

Results

• Sig increase in gas use & $$ electric saving with efficient electric motor (ECM) vs conventional PSC in continuous circulation warm air system

• $ saving and reduction in air conditioning requirement with efficient ECM motor

• Now looking at replacing PSC pump motors with brushless DC units

What about two-stage furnaces ?

Lower speed in ducts can result in

“better” heat exchange to duct, resulting in greater

heat loss, less heat to registers &

increased t/s &furnace operation

Dual Firing Rate Furnaces at Low Fire

What about two-stage condensing furnaces ?

• Compared to condensing furnace with PSC circ fan motor, 2-stage furnace with ECM gave increased gas use

• Increased operation time (~ doubled) may lead to sig. increase in electrical use, esp if PSC motor used with ID fan

• Loss of heat to distribution system may give less heat to some registers and result in homeowner upping the thermostat

• No evidence of gas savings, in spite of claims• May give increased comfort• May be only way of getting ECM fan motor

What about two-stage mid-efficiency furnaces ?

• Compared to furnace with PSC circ fan motor, 2-stage furnace with ECM gave increased gas use

• Increased operation time (~ doubled) may lead to significant increase in electrical use if PSC motor used

• If PSC used for circ fan, major electricity cost increase• Loss of heat to distribution system may give less heat

to some registers and result in homeowner upping the thermostat

• If efficiency maintained on low fire, high risk of condensation and corrosion of primary heat exchanger and vent system

• 2-Stage mid-efficiency furnaces NOT recommended

Gas Boilers

Remember the Conventional Gas Furnace with natural draft and

dilution air

Did NOT met AFUE std and has been off the market for 15 years!

“Antique” Conventional Boilers (natural draft) are still available !

(at low real efficiencies)

Dilution Air

Combustion Air

Newer Gas-Fired Boilers

Major Advances in Gas Burners- High modulation- Power Burners

The New Gas Burner World

• Power burners (forced draft)• Low excess air (EA)• Good modulation with EA control• Low NOx• Usually on boilers, not furnaces• Provide a more assured pathway to high

efficiency condensing• Gun, metal matrix or ceramic (cylinder,

cone, plate

Recent Major Advances in Gas Boilers

Can a condensing boiler condense?

• Yes, if radiant floor• Likely, if fan coil• Difficult but possible if convectors or

radiators and conventional hydronic, as with . . .

Dewpoints for Different Fuels

60708090

100110120130140

0 50 100

Natural GasPropaneNo. 2 Oil

oF

Excess Air, %

Ensuring Condensing Boiler Efficiency

Outdoor reset to lower supply temp.

Increased heat exchange

Preheat service water

Condensing Boiler

Return water below dewpoint

Condensing Boiler

Increase Heat Exchange Surface Preheat Ventilation Air

Preheat Service Water

Get return water below flue gas dewpoint !

Preheat Service Water

Lower Flow Rate

Lower Flow Rate

Gas Water Heaters

Conventional GasWater Heater

• Naturally aspirating• Requires a chimney • Continuous pilot (energy

waste)• Draft hood (extra heated air

loss)• Highly susceptible to spillage• Large off-cycle loss• Low seasonal efficiency ( ~

55% )• Should be an “antique” !

Power-Vented Water Heaters

• Pilot light or not• Draft hood• High on- and off-

cycle losses• High resistance to

depressurization• Minimal efficiency

improvement

Condensing Tank Water Heater

> 90% efficient, as mains water

provides driving force for

condensing

Tankless Water Heaters

Wide range of technologies and efficiencies

Examining performance in lab and field (along with hot water use trial) to optimize performance and develop

appropriate seasonal efficiency test procedure including cold ambients and short cycling

Natural Gas vs Propane

• Natural gas has a higher hydrogen content, so produces more water vapour

• Higher dewpoint with natural gas, so easier to condense than propane

• Propane condensing furnaces less efficient than NG

• Propane mid-efficiency furnaces or boilers more efficient than natural gas

• Propane boilers even more difficult to condense than natural gas

CombinedSpace-Water

Heating Systems

CCHT

Performance of Combo Systems in “almost” real-life conditions

Efficiency of Tank-Based Combo’s vs Outdoor Temperature

Efficiency with Tankless Combo

Inefficient “Instantaneous”Segregated, Non-Condensing

Boiler with Modulation

SU02, Mixed

020406080

100120140

13:4

713

:53

13:5

814

:03

14:0

714

:12

14:1

714

:22

14:2

714

:32

14:3

714

:42

14:4

7

Time

Tem

p [C

]

010203040506070

Con

c [%

]

Flue Gas TO2CO2

Flue Gas Analysis

Efficient Low Mass Condensing Boilers with Segregated Tap water

Warm Air Systems• Proper duct design• Ducts inside house envelope• Ducts well-sealed (mastic)• Adequate returns (ALL bedrooms)• Insulate basement duct runs• Facilitates condensing op (gas only)• Rapid thermal response (setback)• Allows whole-house ventilation• Consider small dia, hi-vel ducts

Hydronic Systems

• High mass, less temperature fluctuations• Many find greater comfort• Not subject to “heat” leakage• Pipes should be insulated• Less suitable for setback savings• Difficult to have condensing (gas)• Can be adjusted for outside temperature

(outdoor reset) for some boilers• Require additional means for ventilation

Summary• With oil, high static burners in efficient boiler systems,

and the option of heating service water externally with excess heat, offers high potential for efficiency and customer satisfaction

• With gas, central condensing furnaces or boilers + fan coils, both with EE motors, are attractive.

• Advanced, modulating burners are making condensing gas boilers attractive, if care is taken to ensure the total system allows condensation.

• For space heaters, advanced combustion (or pellet) woodstoves & fireplaces, and high-P.4 DV gas fireplaces are superior options

• Efficient integrated systems are the way of the future

Review of Objectives• Discuss new space & water heating

technologies for oil, gas and wood• Understand what makes heating systems

efficient• Appreciate advantages & disadvantages of

various new “high efficiency” systems• Be better able to assess suitability of newer

technologies for comfort andhigh efficiency operation in new or existing housing

Heating Publications

http://energy-publications.nrcan.gc.ca/index_e.cfm

under Consumers:Heating, Cooling & Ventilation

affordablecomfort.org

Pittsburgh, PA

2008

If further questions,

contact me (Skip Hayden)Advanced Combustion Technologies,

tel: (613) 996 3186fax: (613) 992 9335

e-mail: skip.hayden@nrcan.gc.ca

AIMS: www.eKOCOMFORT.com

Warm Air Systems• Proper duct design• Ducts inside house envelope• Ducts well-sealed (mastic)• Adequate returns (ALL bedrooms)• Insulate basement duct runs• Facilitates condensing op (gas only)• Rapid thermal response (setback)• Allows whole-house ventilation• Consider small dia, hi-vel ducts

Hydronic Systems

• High mass, less temperature fluctuations• Many find greater comfort• Not subject to “heat” leakage• Pipes should be insulated• Less suitable for setback savings• Difficult to have condensing (gas)• Can be adjusted for outside temperature

(outdoor reset) for some boilers• Require additional means for ventilation

The potential is there.The fun is there.

The comfort is there.

Take it !!!